Piezoelectric actuator

ABSTRACT

A piezoelectric actuator is formed like a rectangular flat plate, and includes a substrate layer, a lower electrode layer, a piezoelectric layer, and an upper electrode layer formed in this order from bottom to top in a thickness direction. The upper electrode layer is constituted of a plurality of electrode segments separated in a surface direction, and connection wires connecting the electrode segments which are adjoining in the surface direction.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a piezoelectric actuator to beinstalled in a light deflector or the like.

2. Description of the Related Art

Japanese Patent Application Laid-Open No. 2016-219603 describes apiezoelectric actuator to be installed in a light deflector of microelectro mechanical systems (MEMS). The piezoelectric actuator is formedlike a rectangular flat plate and includes a substrate layer, a lowerelectrode layer, a piezoelectric layer, and an upper electrode layer,which are formed in this order from bottom to top in the thicknessdirection.

In a piezoelectric actuator, the piezoelectric layer has a void orcrack, a foreign matter (contaminant) is mixed in the piezoelectriclayer, or further, a surface of the piezoelectric layer has a small dentor projection in some cases. These defects in the piezoelectric layertend to lead to a breakdown of the piezoelectric layer in apiezoelectric actuator, in which an electric field (applied voltage) of,for example, 10 V/μm or more is applied to the piezoelectric layer.

Focusing on the fact that the breakdown of the piezoelectric layer tendsto occur along column crystals of a piezoelectric, the piezoelectricactuator described in Japanese Patent Application Laid-Open No.2016-219603 has an electrically conductive thin film placed in betweenthe lower electrode layer and the upper electrode layer, and the columncrystals of the piezoelectric layer are formed in steps, dividing thecolumn crystals into those on the lower side and the upper side of theelectrically conductive thin film. With this arrangement, one columncrystal does not continue in between the lower electrode layer and theupper electrode layer, thus forming two discrete column crystals belowand above the electrically conductive thin film. This suppresses thebreakdown of the piezoelectric layer.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a piezoelectricactuator capable of blocking the propagation of a breakdown of apiezoelectric layer to a surrounding area thereof so as to minimize thefunctional loss of the piezoelectric layer by a unique configuration incase of the occurrence of the breakdown of the piezoelectric layer.

A piezoelectric actuator in accordance with the present invention is apiezoelectric actuator which is formed like a rectangular flat plate andin which a substrate layer, a lower electrode layer, a piezoelectriclayer, and an upper electrode are formed in this order from bottom totop in a thickness direction, wherein the upper electrode layer isconstituted of a plurality of electrode segments separated in a surfacedirection, and connection wires connecting the electrode segments whichare adjoining in the surface direction.

A first breakdown of the piezoelectric layer in a piezoelectric actuatortends to take place, in the surface direction, at a position in thesurface direction where a defective part of the piezoelectric layerexists, and at the time of a breakdown, a high current is generated thatpasses through, in the thickness direction, the position in the surfacedirection. At this time, the high current leaks to the surrounding areain the vicinity of the position in the surface direction, so that thesurrounding area also incurs the breakdown, and the breakdown propagatesin a chain-reaction manner to the surrounding area in succession in thesurface direction, thus expanding at once in the surface direction.

According to the present invention, the upper electrode layer isseparated into a plurality of electrode segments in the surfacedirection. Therefore, even if a breakdown occurs, causing a high currentto pass between the upper electrode layer and the lower electrode layerand the high current to propagate in succession to the surrounding areain the surface direction from a first breakdown position, thepropagation range in the surface direction will be limited to anelectrode segment immediately above a place of the breakdown of thepiezoelectric layer. As a result, even if a breakdown takes place in apiezoelectric layer, the propagation of the breakdown to the surroundingarea can be blocked, thus making it possible to minimize the functionalloss of the piezoelectric layer.

Preferably, in the piezoelectric actuator according to the presentinvention.

the upper electrode layer is separated into a plurality of the electrodesegments in both longitudinal and lateral directions of the surfacedirection, and

each electrode segment is connected through the connection wires toelectrode segments that are adjoining in both directions.

A drive voltage (an applied voltage difference of the piezoelectriclayer) is supplied to each electrode segment through a connection wirefrom an adjoining electrode segment on an upstream side in the directionin which an applied voltage is supplied. Hence, if the separation to aplurality of electrode segments is performed only in one of thelongitudinal direction and the lateral direction of the surfacedirection, then even if the spreading of a breakdown in the surfacedirection can be suppressed, power feeding to the electrode segments ona downstream side in a power feeding direction from the electrodesegment that has incurred a breakdown will be inconveniently cut offwhen resuming the use thereafter.

On the other hand, according to the configuration, the upper electrodelayer is separated into a plurality of electrode segments in bothlongitudinal and lateral directions, so that power will be supplied tothe electrode segments located on the downstream side in the feedingdirection from the electrode segment, which has incurred a breakdown,through a path constituted of a plurality of electrode segments free ofa breakdown.

Preferably, in the piezoelectric actuator according to the presentinvention, each electrode segment has the same size and shape.

The configuration described above enables the upper electrode layer tohave a simpler structure. The sizes and shapes of the electrode segmentscan be set to be different according to a position in the surfacedirection.

Preferably, in the piezoelectric actuator according to the presentinvention, the upper electrode layer is formed of a material that has ahigher melting point than that of the piezoelectric layer.

If the material of the upper electrode layer melts due to a high currentat the time of a breakdown, then the melted material flows into a crackin the piezoelectric layer caused by the breakdown or penetrates intothe piezoelectric layer in some cases. This would cause a short-circuitpath between the upper electrode layer and the lower electrode layer,thus making it difficult to resume the use of the piezoelectric actuatorafter the breakdown.

On the other hand, according to the configuration described above, atthe time of a breakdown, the piezoelectric layer will be cut off firstby melting, thus preventing the short circuit between the upperelectrode layer and the lower electrode layer caused by the melting ofan electrode segment.

Preferably, in the piezoelectric actuator according to the presentinvention, the piezoelectric layer is comprised of a plurality ofpiezoelectric segments which are separated in the surface direction andeach of which is formed immediately below each electrode segment.

According to the configuration, the piezoelectric layer is alsoseparated into a plurality of piezoelectric segments in the surfacedirection. This restrains a high current from propagating in the surfacedirection in the piezoelectric layer at the time of a breakdown.

Preferably, in the piezoelectric actuator according to the presentinvention, the electrode segments and the connection wires are formed ofthe same material.

The configuration enables simpler fabrication of the upper electrodelayer.

Preferably, in the piezoelectric actuator according to the presentinvention, each connection wire has a fuse function that disconnects theconnection wire in response to an energizing current of a specifiedvalue or more.

According to the configuration, at the time of a breakdown of thepiezoelectric layer, a connection wire will be disconnected, thuspreventing the power from being supplied to the electrode segmentimmediately above the location where the breakdown has taken place inthe piezoelectric layer. This makes it possible to instantly end theshort circuit between the upper electrode layer and the lower electrodelayer.

Preferably, the piezoelectric actuator according to the presentinvention includes a power feeding layer that extends in thelongitudinal direction in a width within a single electrode segment inthe lateral direction, the power feeding layer being on the upperelectrode layer, wherein the power feeding layer is connected toelectrode segments at intervals in an array of the electrode segmentsarranged in line on the lower side.

According to the configuration, power is supplied to the plurality ofelectrode segments placed, being distributed in the longitudinaldirection of the upper electrode layer, from the power feeding layer atappropriate intervals in the longitudinal direction. This makes itpossible to suppress a voltage drop in the longitudinal direction in theupper electrode layer. Further, the width of the power feeding layer issmaller than the width of an electrode segment, and the power feedinglayer is within a single electrode segment without overlapping aplurality of electrode segments in the lateral direction. Hence, even ifa high current flows due to a breakdown of the piezoelectric layerimmediately under an electrode segment to which the power feeding layeris connected, the high current will be prevented from reaching theelectrode segments that are adjoining in the lateral direction throughthe power feeding layer. Further, the power feeding layer is connectedto the electrode segments at intervals in the longitudinal direction.Therefore, even if a high current flows due to a breakdown of thepiezoelectric layer immediately wider the electrode segment to which thepower feeding layer is connected, the high current will be preventedfrom reaching the electrode segments that are adjoining in thelongitudinal direction through the power feeding layer.

Preferably, in the piezoelectric actuator according to the presentinvention, the power feeding layer functions also as a power feedingwire through which power supplied from one end in the longitudinaldirection is supplied to another piezoelectric actuator connected to theother end in the longitudinal direction.

A plurality of piezoelectric actuators are used by being connected inseries in some cases. In such cases, each piezoelectric actuator willneed a power feeding wire through which a drive voltage is supplied tothe downstream side. On the other hand, the foregoing configurationenables the power feeding wire to be used to distribute power to theelectrode segments, thus simplifying the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fullyfrom the following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a front view of a light deflector;

FIG. 2A is a plan view of a cantilever of a comparative example;

FIG. 2B is a sectional view taken on line 2B-2B of FIG. 2A;

FIG. 3A is a plan view of a cantilever of an embodiment;

FIG. 3B is a sectional view taken on line 3B-3B of FIG. 3A;

FIG. 4A to FIG. 4C are plan views of the cantilevers of variousembodiments;

FIG. 5A is a plan view of a cantilever in which a piezoelectric layer isconstituted of a plurality of piezoelectric segments;

FIG. 5B is a sectional view taken on line 5B-5B of FIG. 5A;

FIG. 6A is a plan view of a cantilever provided with a power feedinglayer; and

FIG. 6B is a sectional view taken on line 6B-6B of FIG. 6A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theillustrated embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

(Configuration of the Light Deflector)

FIG. 1 is a front view of a light deflector 1. The light deflector 1 isfabricated as MEMS. With respect to the light deflector 1 shaped like aflat plate, the side on which the reflecting surface of a mirror unit 2can be visually recognized will be referred to as “the front surface”and the opposite side therefrom will be referred to as “the backsurface.”

Hereinafter, the top, bottom, left and right of the light deflector 1will mean the top, bottom, left and right in the front view of the lightdeflector 1. For the convenience of understanding the configuration ofthe light deflector 1, a three-axis Cartesian coordinates is defined inFIG. 1 and the like. An origin O is set at the center of the mirror unit2. A z-axis corresponds to the thickness direction of the lightdeflector 1. An X-axis and a Y-axis correspond to a horizontal directionand a vertical direction, respectively. The Z-axis denotes the directionfrom the back surface toward the front surface. The X-axis denotes thedirection from the left toward the right. The Y-axis denotes thedirection from the bottom toward the top. Hereinafter, the configurationof the light deflector 1 will be described on the assumption that themirror unit 2 is in a stationary state, i.e. the normal line of thereflecting surface of the mirror unit 2 is in agreement with the Z-axis.

The light deflector 1 includes the mirror unit 2, inner piezoelectricactuators 3 a, 3 b, a movable frame 4 serving as a movable supportsection, outer piezoelectric actuators 5 a, 5 b, and a fixed frame 6.The fixed frame 6 has a horizontally long rectangular shape when viewedfrom the front side. The long sides and the short sides of the fixedframe 6 are parallel to the X-axis and the Y-axis, respectively.

Referring to FIG. 1 , axes Lx and Ly denote the two axes around whichthe mirror unit 2 rotates in a reciprocating manner (forward and reverserotation). The axes Lx and Ly orthogonally intersect at the center (theorigin O) of the mirror unit 2. The inner piezoelectric actuators 3 a, 3b receive a first drive voltage from a drive unit (not illustrated) andcause the mirror unit 2 to rotate at a first frequency (e.g. 30 kHz)around the axis Ly in the reciprocating manner. The outer piezoelectricactuators 5 a, 5 b receive a second drive voltage from the drive unitand cause the mirror unit 2 to rotate at a second frequency (e.g. 60 Hz)around the axis Lx in the reciprocating manner.

The inner piezoelectric actuators 3 a, 3 b are constituted of acantilever with a piezoelectric structure and configured to besymmetrical with respect to the Y-axis when viewed from the front side.The inner piezoelectric actuators 3 a, 3 b are interconnected at bothend parts in a Y-axis direction, and forms, as a whole, an ellipse ringthat is vertically long in the Y-axis direction, encompassing the mirrorunit 2. The movable frame 4 is formed to be an elliptically contouredannular frame, the inner and outer peripheries thereof being verticallylong in the Y-axis direction, and encompasses the ellipse ringconstituted of the inner piezoelectric actuators 3 a, 3 b on the innerperiphery side thereof.

Torsion bars 21 a, 21 b vertically and linearly project along the Y-axisfrom the mirror unit 2, connect to the junction of the innerpiezoelectric actuators 3 a, 3 b at a middle part, and connect to theinner periphery of the movable frame 4 at the projection ends. The axisLy coincides with the centerlines of the torsion bars 21 a, 21 b.

The outer piezoelectric actuators 5 a, 5 b are provided on the innerperiphery side of the rectangular fixed frame 6 and placed symmetricallywith respect to the movable frame 4 in the X-axis direction. Each of theouter piezoelectric actuators 5 a, 5 b is constituted of a plurality ofcantilevers 23 connected in series in a meander arrangement. As with thecantilevers (no reference numerals) of the inner piezoelectric actuators3 a, 3 b, the cantilevers 23 are also provided with the piezoelectricstructures. Each of the cantilevers 23 can be used alone as apiezoelectric actuator.

To be specific, the cantilevers 23 are arranged in line in the X-axisdirection, with the longitudinal direction thereof being the Y-axisdirection. The plurality of cantilevers 23 are connected to thecantilevers 23 that are adjoining on the right or left in the lateraldirection (the X-axis direction) at the end parts in the longitudinaldirection (the Y-axis direction) via folded-back parts (no referencenumerals).

The cantilevers 23 at both ends in the X-axis direction in each of theouter piezoelectric actuators 5 a, 5 b have lengths that are half thelengths of the remaining cantilevers 23, and are connected to the fixedframe 6 and the movable frame 4 on the Y-axis. The end parts of thecantilevers 23 connected to the fixed frame 6 constitute the proximalend parts of the outer piezoelectric actuators 5 a, 5 b, and the endparts of the cantilevers 23 connected to the movable frame 4 constitutethe distal end parts of the outer piezoelectric actuators 5 a, 5 b.

Electrode pads 16 a, 16 b are provided, each in a plural number, on thesurface of each short-side part of the fixed frame 6. The electrode pads16 a are connected to the inner piezoelectric actuator 3 a and the outerpiezoelectric actuator 5 a on the left half section of the lightdeflector 1 through internal wiring of the light deflector 1. Theelectrode pads 16 b are connected to the inner piezoelectric actuator 3b and the outer piezoelectric actuator 5 b on the right half section ofthe light deflector 1 through the internal wiring of the light deflector1.

(Operation of the Light Deflector)

The operation of the light deflector 1 will be described. Hereinafter,when there is no particular need for discriminating between the innerpiezoelectric actuators 3 a and 3 b, a generic term “the innerpiezoelectric actuators 3” will be used. When there is no particularneed for discriminating between the outer piezoelectric actuators 5 aand 5 b, a generic term “the outer piezoelectric actuators 5” will beused. When there is no particular need for discriminating between theelectrode pads 16 a, 16 b, a generic term “the electrode pads 16” willbe used.

The light deflector 1 is installed as a two-dimensional scanner in avideo device, a vehicular headlight or the like. The light deflector 1is housed in a package, and the electrode pads 16 of the light deflector1 and the terminals of the package are connected by bonding wires (notillustrated). A drive voltage is supplied to the inner piezoelectricactuators 3 and the outer piezoelectric actuators 5 from the electrodepads 16 through piezoelectric layers 38 (FIG. 3B).

Light (e.g. a laser beam) from a light source (e.g. a semiconductorlaser light source), which is not illustrated, enters at the center (theorigin O of a three-axis coordinate system) of the mirror unit 2 of thelight deflector 1.

The outer piezoelectric actuators 5 operate on the drive voltage fromthe electrode pad 16 to cause the movable frame 4 to rotate around theX-axis at the second frequency in the reciprocating manner. This causesthe minor unit 2 to rotate around the axis Lx in the reciprocatingmanner at the second frequency. The axes Lx and the X-axis do notcoincide with each other. This is because, since the mirror unit 2 isrotating around the axis Ly in the reciprocating manner, the axis Lxalso moves as the reciprocating rotation of the axis Ly, as will bediscussed hereinafter. In contrast thereto, the X-axis remains stillwith respect to the fixed frame 6.

The operation of the outer piezoelectric actuators 5 will be describedin more detail. Each of the outer piezoelectric actuators 5 is formed ofa plurality of cantilevers 23 placed in the meander arrangement. Whenthe cantilevers 23 are numbered in a sequential order at the junctionends from the proximal end (on the fixed frame 6 side) to the distal end(on the movable frame 4 side) of the outer piezoelectric actuator 5, theodd-numbered cantilevers 23 and the even-numbered cantilevers 23 receivethe drive voltages of the same frequency and opposite phases, thusdeforming such that the projecting directions of the deformation curvesbecome opposite.

As a result, among the cantilevers 23 placed in the meander arrangement,the cantilevers 23 adjoining to each other in the X-axis direction curvein the opposite directions when the outer piezoelectric actuators 5operate. At this time, the stored amount of the relative rotation amountof the distal end part with respect to the proximal end part of each ofthe cantilevers 23 is the amount of rotation (the amount of torsion) ofthe inner piezoelectric actuators 3 around the axis Lx with respect tothe outer piezoelectric actuators 5.

Meanwhile, the inner piezoelectric actuators 3 cause the torsion bars 21to rotate, in the reciprocating manner, around the axis Ly serving asthe central axis at the first frequency by the first drive voltage fromthe electrode pads 16. The first frequency is set at the resonantfrequency of the minor unit 2 around the axis Ly so as to secure a highfrequency. The second frequency, which is the frequency of thereciprocating rotation of the mirror unit 2 around the axis Lx is set toa non-resonant frequency.

Thus, the minor unit 2 rotates around the axis Lx at the non-resonantfrequency in the reciprocating manner while rotating around the axis Lyat the resonant frequency in the reciprocating manner. As a result, theminor unit 2 swings to the left and right at the resonant frequency andswings up and down at the non-resonant frequency when viewed from thefront side.

The axis Lx coincides with the X-axis and the axis Ly coincides with theY-axis only when the normal line of the reflecting surface of the mirrorunit 2 coincides with the Z-axis. The light from a light source (notillustrated) is reflected from the center of the minor unit 2 and isemitted as scanning light in a direction corresponding to the angle ofrotation around the axis Lx or Ly on a moment-to-moment basis.

(Piezoelectric Actuator of a Comparative Example)

FIG. 2A is a plan view of a cantilever 123 of a comparative example, andFIG. 2B is a sectional view taken at line 2B-2B of FIG. 2A. Thecantilever 123 of the comparative example is illustrated, being comparedwith the cantilever 23 of the embodiment so as to clarify the featuresof the cantilever 23.

The plan view illustrates the piezoelectric actuator observed from abovein the laminating direction (the thickness direction of thepiezoelectric actuator shaped like a flat plate) and corresponds to aview of the piezoelectric actuator viewed from the front side of thelight deflector 1 in FIG. 1 .

The cantilever 123 is formed of a laminate construction of a substratesection 30 at the bottom and a piezoelectric structure section 31 at thetop. The substrate section 30 serving as a substrate layer isconstituted of only one layer, namely, a silicon (Si) layer 35. The Silayer 35 is a front surface Si layer of a silicon-on-insulator (SOI)plate, which is well known in the fabrication of semiconductors. As iswell known, the SOI plate has a three-layer laminate structureconstituted of a silicon dioxide (SiO₂) layer (not illustrated)sandwiched between the front surface Si layer 35 on the upper side andthe back surface Si layer (not illustrated) on the lower side. After thepiezoelectric structure section 31 is deposited on the Si layer 35 onthe front surface side of the SOI plate, the SOI plate is etched fromthe back surface side (the lower side) to remove the back surface Silayer on the lower side and the SiO₂ layer at the middle. In thecompleted cantilever 123, only one layer, namely, the Si layer 35, isleft. The thickness (the dimension in the Z-axis direction) of the Silayer 35 is, for example, 50 μm.

The piezoelectric structure section 31 has a laminate structure in whicha lower electrode layer 37, a piezoelectric layer 38, and an upperelectrode layer 39 are stacked in this order from bottom to top. Thematerial of the piezoelectric layer 38 is, for example, lead zirconatetitanate (PZT). The method for forming a PZT film includes, for example,a sputtering method, an ion plating method, and a metal organic chemicalvapor deposition (MOCVD) method. Further, a pulse laser deposition (MD)method, a molecular beam epitaxy (MBE) method, a chemical solutiondeposition (CSD) method and a sol-gel method are also available. In theformation process of the piezoelectric layer 38, the continuous columnarcrystals of PZT gradually grow toward the top of the lower electrodelayer 37.

In the cantilever 123, the upper electrode layer 39 continues in thesurface direction of the cantilever 123 shaped like a flat plate. Thelateral direction and the longitudinal direction of the cantilever 123are two directions in the surface direction.

While the light deflector 1 provided with the cantilever 123 is inoperation, the first and the second drive voltages are supplied to thefixed frame 6 from an external drive voltage unit outside the lightdeflector 1. The first and the second drive voltages supplied to thefixed frame 6 are directed between the lower electrode layer 37 and theupper electrode layer 39 of the inner piezoelectric actuators 3 and theouter piezoelectric actuators 5 through the internal wiring of the lightdeflector 1. Then, the first and the second drive voltages are applied,as the applied voltages, between both surfaces of the piezoelectriclayer 38 of each piezoelectric actuator of the inner piezoelectricactuators 3 and the outer piezoelectric actuators 5. The piezoelectriclayer 38 expands and contracts in the longitudinal direction accordingto the applied voltage. With the expansion and contraction, thesubstrate section 30 bends in the thickness direction, and the distalend part in the longitudinal direction of the cantilever 123 isrelatively displaced in the thickness direction with respect to theproximal end part.

In the cantilever 123, a breakdown of the piezoelectric layer 38presents a problem. A breakdown tends to occur in the cantilevers 23 ofthe outer piezoelectric actuators 5, to which a high electric field of10 V/μm or more has to be applied between the both surfaces of thepiezoelectric layer 38 in order to secure a large amount of deflectionalthough a low driving frequency (the foregoing second frequency) isacceptable, as in the case of the cantilever 123. In other words, abreakdown is more likely to occur in the outer piezoelectric actuators 5having the piezoelectric layer 38 to which a higher applied voltage thanthat of the piezoelectric layer of the inner piezoelectric actuators 3is applied.

As already described as a problem with the prior art, it is assumed thatthe piezoelectric layer 38 has incurred a breakdown. For the convenienceof explanation, the position in the piezoelectric layer 38 where a firstbreakdown has occurred is indicated as a trigger position Dt in FIG. 2Aand FIG. 2B. At the trigger position Dt, a high current flows betweenthe lower electrode layer 37 and the upper electrode layer 39. The highcurrent propagates in succession in the surface direction from thetrigger position Dt, easily ending up as a breakdown spreading in thesurface direction. In such a case, the outer piezoelectric actuators 5will be totally disabled. In contrast thereto, according to the lightdeflector 1 of the embodiment, even if a breakdown of the piezoelectriclayer 38 occurs at the trigger position Dt, the propagation of thebreakdown in the surface direction will be controlled to a minimum, aswill be discussed hereinafter in conjunction with FIG. 3A and FIG. 3B.

First Embodiment

FIG. 3A is a plan view of a cantilever 23 a, which is an example of thecantilever 23 of the light deflector 1, and FIG. 3B is a sectional viewtaken on line 3B-3B in FIG. 3A.

Regarding the cantilever 23 a, like constituent elements as theconstituent elements of the cantilever 123 will be assigned likereference numerals assigned to the constituent elements of thecantilever 123. The cantilever 23 a is fabricated from the cantilever123. The cantilever 123 and the cantilever 23 a differ in the structureof a piezoelectric structure section 31. The rest of the construction ofthe cantilever 23 a is the same as that of the cantilever 123. Hence, adescription will be given of the cantilever 23 a, focusing mainly on theconfiguration of the piezoelectric structure section 31.

The piezoelectric structure section 31 of the cantilever 23 a isconstituted of a plurality of electrode segments 43 separated in thesurface direction as the planar direction of two axes, namely, X-axisand Y-axis, and connection wires 44 that connect the electrode segments43 that are adjoining in the surface direction.

The plurality of electrode segments 43 and the plurality of connectionwires 44 in the upper electrode layer 39 of the cantilever 23 a areformed by etching the piezoelectric layer 38 (FIG. 29 ) of thecantilever 123 from the upper side. In this example, therefore, thethicknesses of the electrode segments 43 and the plurality of connectionwires 44 are the same as the thickness of the piezoelectric layer 38 ofthe cantilever 123. Each of the connection wires 44 is formed to have asmall width so as to provide a fuse function (to cut the connectionwires 44 by melting in case of the occurrence of a breakdown), whichwill be discussed hereinafter, but is formed not to be excessivelynarrow.

Each of the electrode segments 43 is formed to be square in a plan view.The connection wires 44 extend between the midpoints of the opposingsides of the electrode segments 43 that are adjoining in the X-axisdirection and the Y-axis direction. Referring to FIG. 3A, the blacklines indicated in the gap between the adjoining electrode segments 43denote the connection wires 44. Each of the connection wires 44 connectsthe electrode segments 43 at both ends.

The operation of the cantilever 23 a will be described. At a firstbreakdown, a high current that penetrates a trigger position Dt in theZ-axis direction appears between a lower electrode layer 37 and an upperelectrode layer 39, as with the cantilever 123. The high current flowsnot only through the trigger position Dt, which is the one location inthe surface direction, but also reaches the surrounding area in contactwith the trigger position Dt in the surface direction.

As a result, the breakdown is apt to propagate in the surface direction,centering around the trigger position Dt, as in the case of thecantilever 123. However, the spreading of the high current in thesurface direction is limited to one lectrode segment 43 in the area inthe surface direction that includes the trigger position Dt in thesurface direction. As a result, the breakdown of the piezoelectric layer38 is restrained to the area in the surface direction of the oneelectrode segment 43 that includes the trigger position Dt in thesurface direction.

Thus, in the cantilever 23 a, the total functional damage will beavoided, although a breakdown may occur due to a defect, such as a void,in the piezoelectric layer 38.

The fuse function of the connection wires 44 will be described. Theconnection wires 44 are formed to have a narrow width in a plan view. Incase of a breakdown of the piezoelectric layer 38, current flows in aconcentrative manner to the electrode segment 43 immediately above thetrigger position Dt (hereinafter referred to as “the trigger electrodesegment 43 t”) from the surrounding area through the connection wires44. Hence, an energizing current of a specified value or more passesthrough the connection wires 44 extending from the trigger electrodesegment 43 t of the trigger position Dt (hereinafter referred to as “thetrigger connection wires 44 t”). Further, in case of a breakdown of thepiezoelectric layer 38 caused by a high current, high heat is generatedat the trigger position Dt. Thus, the trigger connection wires 44 t areinstantly cut by melting by the heat generated due to the energizingcurrent of the specified value or more and/or the high heat propagatedfrom the trigger position Dt.

As a result, the supply of current to the trigger electrode segment 43 tis cut off, instantly ending the breakdown of the piezoelectric layer38. In other words, the propagation of the breakdown in the surfacedirection in the piezoelectric layer 38 is suppressed.

Each of the electrode segments 43 is connected through the connectionwires 44 to the electrode segments 43 adjoining thereto in bothlongitudinal and lateral directions. Therefore, each of the electrodesegments 43 can receive the drive voltage supplied through theconnection wires 44 shared with other adjoining non-trigger electrodesegments 43 even if the connection, through the trigger connection wire44 t, with the trigger electrode segment 43 t is cut off due to theelectrode segment 43 adjoining thereto being the trigger electrodesegment 43 t.

Other Embodiments

FIG. 4A to FIG. 4C illustrate cantilevers 23 b to 23 d, which arevarious embodiments of the cantilever 23 of FIG. 1 . For the cantilevers23 b to 23 d, the longitudinal sectional views thereof are omitted, andonly the plan views thereof are presented. Only the differences from thecantilever 23 a will be described. Hereinafter, cantilevers 23 a to 23 f(the cantilevers 23 e and 23 f being illustrated in FIG. 5A to FIG. 6B)will be generically referred to as “the cantilevers 23” unless there isa need to discriminate between these cantilevers.

The cantilever 23 b of FIG. 4A is provided with connection wires 45 inplace of the connection wires 44 of the cantilever 23. In the case ofthe connection wires 44 (FIG. 3A), the midpoints of the opposing sidesof the electrode segments 43 that are adjoining in the surface directionare connected. In comparison, the connection wires 45 of the cantilever23 b connect the opposing vertexes of the electrode segments 43 that areadjoining in the surface direction. Referring to FIG. 4A, each of theconnection wires 45 is provided at the crossroad defined by the vertexesof four electrode segments 43 and connects together the four vertexesbordering on the crossroad.

In the cantilever 23 c of FIG. 4B, electrode segments 48 and connectionwires 49 are formed in place of the electrode segments 43 and theconnection wires 44, respectively, of the cantilever 23 a (FIG. 3A). Theelectrode segments 48 have equilateral triangular shapes in a plan view.In each pair of two electrode segments 48, which are adjoining to eachother in the surface direction and which have their sides opposing toeach other, the connection wires 49 connect the midpoints of theopposing sides. In order to obtain minute filling of the electrodesegments 48, the pairs of the electrode segments 48 having their sidesopposing to each other in the X-axis direction are formed in a diamondshape in a plan view.

In the cantilever 23 d of FIG. 4C, electrode segments 53 and connectionwires 54 are formed in place of the electrode segments 43 and theconnection wires 44, respectively, of the cantilever 23 a (FIG. 3A). Theelectrode segments 53 have regular hexagon shapes in a plan view. Ineach pair of two electrode segments 53, which are adjoining to eachother in the surface direction and which have their sides opposing eachother, the connection wires 54 connect the midpoints of the opposingsides. In order to obtain minute filling of the electrode segments 53,each group consisting of a plurality of electrode segments 53 arrangedwith their centers aligned in the Y-axis direction is staggered by ½pitch in the X-axis direction with respect to a group that is adjoiningin the Y-axis direction. One pitch means the width as a dimension of oneelectrode segment 53 in the X-axis direction.

(Another Structure of the Piezoelectric Layer)

FIG. 5A is a plan view of a cantilever 23 e provided with apiezoelectric layer 60, which is different from the piezoelectric layer38 of the cantilever 23 a, and FIG. 5B is a sectional view taken on line5B-5B of FIG. 5A. The configuration of the cantilever 23 e will bedescribed only as it differs from the configuration of the cantilever 23a.

In the cantilever 23 e, a piezoelectric layer 60 is constituted of aplurality of piezoelectric segments 61, which are separated in thesurface direction and each of which is formed immediately under each ofelectrode segments 43. This means that, in the cantilever 23 e, thepiezoelectric layer 60 as well as an upper electrode layer 39 areseparated in the surface direction.

The electrode segments 43 and the piezoelectric segments 61 that arelocated at the same position in the surface direction (the thicknessdirection) constitute pairs. The electrode segment 43 and thepiezoelectric segment 61 of each pair are integral in the thicknessdirection. Thus, the pairs are separated in both longitudinal andlateral directions within the surface direction, and the pairs that areadjoining in the surface direction are connected through the connectionwires 44 to a plurality of upper electrode layers 39 that are adjoiningin both directions.

As a result, the spreading of a high current in the surface direction atthe start of a breakdown to the range of the piezoelectric layer 60 aswell as the range of the upper electrode layers 39 in the Z-axisdirection will be prevented. Hence, the propagation of the breakdown inthe surface direction can be securely prevented.

(Power Feeding Layer)

FIG. 6A is a plan view of the cantilever 23 f provided with a powerfeeding layer 66, and FIG. 6B is a sectional view taken on line 6B-6B ofFIG. 6A. The configuration of the cantilever 23 f will be described onlyabout a difference from the configuration of the cantilever 23 a of FIG.2B.

In comparison with the piezoelectric structure section 31 of thecantilever 23 a, a piezoelectric structure section 31 of the cantilever23 f has additional two layers, namely, an insulating layer 65 and thepower feeding layer 66 stacked in this order from the bottom, which aredeposited on the upper side of an upper electrode layer 39. The planview of FIG. 6A omits illustrating the insulating layer 65, so that theinternal structure is visible. The insulating layer 65 covers the entirefront surface (the upper surface) of the upper electrode layer 39.

The power feeding layer 66 extends in the Y-axis direction, the widththereof being within the inner side of a single electrode segment 43 inthe X-axis direction. The power feeding layer 66 is connected to theelectrode segments 43 at intervals in the array of the electrodesegments 43 arranged in line in the Y-axis direction on the lower side(hereinafter referred to as “the power feeding array 71”).

Hereinafter, in the cantilever 23 (the generic term of the cantilevers23 a to 23 f), a common (the same potential) drive voltage is requiredto be supplied to the electrode segments 43 of the upper electrode layer39. However, the connection wires 44 are narrow, thus having highresistance. This causes a voltage drop in a power feeding direction(toward the other end side, which is the distal end side that is farfrom one end, which is the proximal end closer to electrode pads 16),inconveniently leading to a drop in the drive voltage supplied to theelectrode segments 43 on the downstream side in the power feedingdirection.

The outer piezoelectric actuators 5 of the light deflector 1 need a wirefor supplying the drive voltage to each of the plurality of (two ormore) cantilevers 23 in the meander arrangement. The power feeding layer66 serves also as the wire.

In the cantilever 23 f, power is supplied to the plurality of electrodesegments 43, which are placed by being distributed in the longitudinaldirection of the upper electrode layer 39, from the power feeding layer66 at appropriate intervals in the longitudinal direction (at thelongitudinal intervals of via holes 69). The power feeding layer 66 issufficiently wide with respect to the connection wires 44, so that thesame drive voltage that has not dropped is supplied along a powerfeeding array 71 to the electrode segments 43 connected to the powerfeeding layer 66 (hereinafter referred to as “the directly connectedelectrode segments 43”). Further, the electrode segments 43 not directlyconnected to the power feeding layer 66 (hereinafter referred to as “theindirectly connected electrode segments 43”) receive the drive voltagefrom one or two directly connected electrode segments 43 that are theclosest ones following an n (n≥0) number or more indirectly connectedelectrode segments 43 from the directly connected electrode segments 43.Further, the maximum value of n is set to be sufficiently small, so thata drop in the drive voltage in the indirectly connected electrodesegments 43 will be small. Thus, in the cantilever 23 f, the drivevoltage free of a voltage drop can be supplied to all the electrodesegments 43.

In the cantilever 23 f, the width of the power feeding layer 66 issmaller than the width of the electrode segments 43, and the powerfeeding layer 66 is accommodated within a single electrode segment 43 inthe X-axis direction. As a result, when the directly connected electrodesegments 43 become the trigger electrode segments 43 t, even if a highcurrent flows into the directly connected electrode segments 43, thehigh current is prevented from spreading through the power feeding layer66 to the indirectly connected electrode segments 43 adjoining in thedirection of the piezoelectric actuator.

Further, in the cantilever 23 f, the power feeding layer 66 is connectedto the electrode segments 43 of the power feeding array 71 at intervalsin the longitudinal direction. With this arrangement, even if thedirectly connected electrode segments 43 to which the power feedinglayer 66 is connected become the trigger electrode segments 43 t,causing a high current to flow into the directly connected electrodesegments 43, the high current will be prevented from spreading throughthe power feeding layer 66 to the electrode segments 43 that areadjoining in the power feeding array 71.

(Examples of Values)

In contrast to the upper electrode layer 39 of the cantilever 123, whichis formed of a single flat layer, covering substantially the entiresurface of the piezoelectric layer 38, the upper electrode layer 39 ofthe cantilever 23 is separated into the plurality of electrode segments43, 48 or 53 (hereinafter referred to as “the electrode segments 43 orthe like”), which are separated in the surface direction, and the totalarea of the electrode segments 43 or the like will be the area to whichan applied voltage is applied in the piezoelectric layer 38. Each of theelectrode segments 43 or the like will be a minimum breakdown unit inthe cantilever 23 in case of a breakdown of the piezoelectric layer 38,thereby restricting the breakdown in the cantilever 23 to a singleelectrode segment 43 or the like. For this reason, the dimensions of theelectrode segments 43 or the like are preferably smaller. For example,the area of each of the electrode segments 43 or the like is set to be10% or less of the area of the entire region where the upper electrodelayer 39 is formed or the area of the entire piezoelectric layer 38.

If the area of the entire region where the upper electrode layer 39 isformed is set to 5 mm², then each of the electrode segments 43, whichhave a square shape in a plan view, is set to 100 μm×100 μm. This meansthat “the area of the electrode segment 43/the total area of the upperelectrode layer 39≈0.2%.” Setting to these values guarantees that thecantilever 23 as a whole can be used without trouble even if thepiezoelectric layer 38 corresponding to one electrode segment 43 or thelike incurs a malfunction in the cantilever 23.

The gap between the electrode segments 43 or the like that are adjoiningin the surface direction (e.g. the length of the connection wires 44) ispreferably small. However, in order to accomplish a stable lithographyprocess, the dimension of the gap is set such that the electrodesegments 43 or the like that are adjoining in the surface direction areplaced apart in the surface direction by at least 5 μm or more accordingto current technology. Hence, in the electrode segment 43, which issquare in a plan view, or the electrode segment 48, which isequilateral-triangular in a plan view, the length of one side ispreferably set to be 20 μm or more for the total area of the electrodesegments 43 or the like to secure 60% or more of the area of the entireregion where the upper electrode layer 39 is formed (hereinafterreferred to as “the upper electrode covering percentage”). In thecantilever 23 d, in which the electrode segments 53 have regular hexagonshapes in a plan view, one side of the regular hexagon shape ispreferably set to 12 μm or more to secure the same upper electrodecovering percentage.

As described above, the connection wires 44, 45, 48 and 54 (hereinafterreferred to as “the connection wires 44 or the like”) will be cut bymelting attributed to the heat generated by concentrated current if abreakdown occurs in a region of the piezoelectric layer 38 immediatelybelow one electrode segment 43 or the like to which the connection wiresare connected. For this reason, the width of the connection wires or thelike is preferably set to 20 μm or less. By providing the connectionwires 44 or the like with a fuse function that disconnects theconnection wires 44 if a current of a specified value or more passestherethrough, the breakdown current can be instantly stopped in case ofa breakdown, thus restraining damage to the piezoelectric layer 38caused by the breakdown.

The material of the upper electrode layer 39 preferably has a meltingpoint that is higher than the material of the piezoelectric layer 38.This is because, if the melting point of the upper electrode layer 39 islower than that of the piezoelectric layer 38, then the melted materialof the upper electrode layer 39 may flow into a crack in thepiezoelectric layer 38 caused by a breakdown in case of piezoelectricdestruction, possibly leading to a short circuit between the lowerelectrode layer 37 and the upper electrode layer 39. To avoid this, theupper electrode layer 39 should not be made excessively thick. It isdesirable, therefore, to form the upper electrode layer 39 by a thinfilm using a type of material which has a melting point that is higherthan that of the material constituting the piezoelectric layer 38. Morespecifically, the material of the upper electrode layer 39 is, forexample, Pt, Ti or Ir, and the upper electrode layer 39 is formed tohave a layer thickness of 200 nm or less.

The width of the power feeding layer 66 is preferably minimized. Forexample, the ratio of the width of the power feeding layer 66 to thewidth of the electrode segment 43 is desirably set to ⅓ or less andfurther desirably set to 1/10 or less. However, the width is set to besufficiently larger than the connection wire 44 or the like providedwith the fuse function.

(Modification Examples)

In the cantilever 23 a, the upper electrode layer 39 has a plurality ofelectrode segments 43 separated in both the X-axis direction and theY-axis direction. According to the present invention, however, theseparation may be only in one of the X-axis direction and the Y-axisdirection.

The description has been given about the cantilever 23 of the outerpiezoelectric actuators 5 in conjunction with FIG. 3A to FIG. 6B.However, the separation structure of the piezoelectric layer 38 in thecantilever 23 is not limited to a piezoelectric actuator having therectangular flat-plate-like shape, as with the outer piezoelectricactuators 5. The piezoelectric actuator in accordance with the presentinvention is applicable also to a piezoelectric actuator having a curved(e.g. elliptical) flat plate shape in a plan view, as with the innerpiezoelectric actuators 3. In the piezoelectric actuator having thecurved flat plate shape, the longitudinal direction is the direction inwhich the piezoelectric actuator extends lengthwise (e.g. the directionin which an elliptical contour line extends). Further, the widthdirection is the lateral direction.

In the embodiments, the square, the equilateral triangular or theregular hexagonal shapes in a plan view have been illustrated as theminute structures of the electrode segments of the upper electrode layer39. According to the present invention, minute structures of othershapes can be also adopted.

In the embodiments, a thickness Ha of the piezoelectric layer 38 in theform of the continuous flat layer before separation in the cantilever123, a thickness Hb of the electrode segments, such as the electrodesegments 43 or the like, and a thickness Hc of the connection wires,such as the connection wires 44 or the like, have a relationship denotedby Ha=Hb=Hc. According to the present invention, the relationship may beHa=Hb>Hc insofar as the fuse function of the connection wires in case ofa breakdown is secured. Further, in the case where Hb>Hc, the fusefunction of the connection wires can be secured by appropriatelyincreasing the width of the connection wires.

It will be appreciated by persons skilled in the art that the disclosedtechnique is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the disclosed technique isdefined only by the claims, which follow.

What is claimed is:
 1. A piezoelectric actuator which has a rectangularflat plate shape and in which a substrate layer, a lower electrodelayer, a piezoelectric layer, and an upper electrode layer are formed inthis order from bottom to top in a thickness direction, wherein theupper electrode layer formed on the piezoelectric layer is constitutedof (i) a plurality of electrode segments separated in a surfacedirection and (ii) connection wires, and wherein the connection wiresconnect respective adjoining pairs of electrode segments which areformed on the piezoelectric layer and that are adjoining in the surfacedirection, and each connection wire is positioned in a gap between itscorresponding adjoining pair of electrode segments.
 2. The piezoelectricactuator according to claim 1, wherein the plurality of electrodesegments are separated in both longitudinal and lateral directions ofthe surface direction, and the connection wires connect the electrodesegments that are adjoining in both the longitudinal and lateraldirections.
 3. The piezoelectric actuator according to claim 2, whereineach electrode segment has a same size and shape.
 4. The piezoelectricactuator according to claim 3, wherein each connection wire has asmaller width than a width of the respective electrode segments alongthe surface direction.
 5. The piezoelectric actuator according to claim2, wherein each connection wire extends between midpoints of opposingsides of its corresponding adjoining pair of electrode segments.
 6. Thepiezoelectric actuator according to claim 5, wherein each connectionwire has a smaller width than a width of the respective electrodesegments along the surface direction.
 7. The piezoelectric actuatoraccording to claim 1, wherein the upper electrode layer is formed of amaterial that has a higher melting point than a material from which thepiezoelectric layer is formed.
 8. The piezoelectric actuator accordingto claim 1, wherein the piezoelectric layer is constituted of aplurality of piezoelectric segments which are separated in the surfacedirection and each of which is formed immediately below a respective oneof the plurality of electrode segments.
 9. The piezoelectric actuatoraccording to claim 1, wherein the electrode segments and the connectionwires are formed of a same material.
 10. The piezoelectric actuatoraccording to claim 1, wherein each connection wire has a fuse functionthat disconnects a connection wire in response to an energizing currentof at least a specified value.
 11. The piezoelectric actuator accordingto claim 1, further comprising: a power feeding layer that extends in alongitudinal direction in a width within a single electrode segment in alateral direction, the power feeding layer being on the upper electrodelayer, wherein the power feeding layer is connected to electrodesegments at intervals in an array of the electrode segments arranged inline on a lower side.
 12. The piezoelectric actuator according to claim11, wherein the power feeding layer functions also as a power feedingwire through which power supplied from one end in the longitudinaldirection is supplied to another piezoelectric actuator connected to theother end in the longitudinal direction.
 13. The piezoelectric actuatoraccording to claim 1, wherein the connection wires are formed on thepiezoelectric layer.
 14. The piezoelectric actuator according to claim13, wherein the plurality of electrode segments are separated in bothlongitudinal and lateral directions of the surface direction, and theconnection wires connect the electrode segments that are adjoining inboth the longitudinal and lateral directions.
 15. The piezoelectricactuator according to claim 14, wherein each connection wire extendsbetween midpoints of opposing sides of its corresponding adjoining pairof electrode segments.
 16. The piezoelectric actuator according to claim15, wherein each connection wire has a smaller width than a width of therespective electrode segments along the surface direction.
 17. A lightdeflector comprising: the piezoelectric actuator according to claim 1;and a rotating mirror driven by the piezoelectric actuator.